The recent influx of counterfeit threaded fasteners into the American economy has caused a multiplicity of problems, according to a report by the Subcommittee on Oversight and Investigations of the Committee on Energy and Commerce, U.S. House of Representatives. Unscrupulous manufacturers, importers and venders have distributed and sold over thirty millions of substandard fasteners in place of standard fasteners. The U.S. industries in the main utilize a standard for steel fasteners published by the Society of Automotive Engineers (SAE), SAE J 429 January 80. (Although this specification refers to the SAE J 429 Standard and cites particular grades of fasteners as examples, it is equally applicable to other fastener standards). As examples, lower grades 8.2 and 5.2 fasteners have been unknowingly bought by dealers and users in place of the higher grades 8 and 5 fasteners, respectively. Many of the lower grade fasteners have been falsely coded with the counterfeit grade identification markings.
The major differences between Grades 8 and 8.2 and 5 and 5.2 fasteners, as specified by SAE J 429 January 80, are in the percent permissible carbon and boron content. The Grades 8 and 5 contain higher amounts of carbon and therefore are of higher quality than Grades 8.2 and 5.2, respectively. Thus, additions of boron to Grades 8 and 5 steels, for the purpose of hardening, is unnecessary. The lower grade steel fasteners, Grade 8.2 and 5.2, have almost identical mechanical properties as the higher grade steel fasteners at ambient temperatures. However, following a high temperature excursion, the lower grade steel fasteners are susceptible to high temperature embrittlement, a condition that has led to catastrophic failures.
An additional difference between the Grade 8 and Grade 8.2 fastener is their heat treatments. Each, however, will conform to the required minimum mechanical properties at room temperature but not at elevated temperature since the Grade 8.2 fastener is of different composition and is tempered at a lower temperature than the Grade 8 fastener. The surface hardness of each grade is almost identical and not amenable to discrimination by hardness testing. Thus, destructively testing by tensile tests above the tempering temperature of the Grade 8.2 fastener is required to definitively determine the grade of each fastener.
Although both grades of fasteners are used at ambient temperature, the Grade 8.2 fastener will prematurely fail, compared to the Grade 8 fastener, due either to its greater susceptibility to stress corrosion cracking or hydrogen embrittlement.
Heretofore, the difference between the higher quality and lower quality fastener grades has been impossible to successfully detect in an accurate manner except by the costly destructive test method already mentioned and defined in SAE J 429 January 80. Thus, it is important that some way be found to detect the bogus fasteners, a way that avoids the usual destructive testing, a way that is nondestructive, fast, simple and accurate. Such a detection method is needed that will stop the illegal sale of bogus fasteners and detect those already invoiced so that they can be sorted and separated from higher grade fasteners and allow each grade to be appropriately classified for future use.
There are several methods of nondestructively discriminating between bodies having similar appearances but of slightly different composition or even of different material. The following paragraphs discuss appropriate examples of these.
In one instance the relatively old technology of eddy current testing technique is utilized to attempt to separate higher grade fasteners from lower grade fasteners. This method principally compares the electrical conductivity, synonymous with thermal conductivity, and magnetic permeability of a resulting read-out wave form of the higher grade standard fastener to that of the sample.
According to SAE J 490C, the standard permissible variations in alloy content between Grade 8 and Grade 8.2 fasteners result in a very small percent difference by weight in the alloying elements. The minimum differences allowed in carbon (C) and iron (Fe) content by percent weight between Grade 8 and Grade 8.2 fasteners, dictated by SAE J 490C. are 0.43 C and 0.28 Fe. Such small differences in these major alloying elements that affect the conductivity and the permeability of the fasteners, result in an estimated difference (by a linear mixture approximation) in thermal conductivity between Grade 8 and Grade 8.2 fasteners of only 0.2%. This difference is very small and out of the sensitivity range of the eddy current technique.
In another instance U.S. Pat. No. 4,255,962, issued to Leland E. Ashman, teaches a method of distinguishing a simulated diamond from a natural diamond by utilizing a probe which applies a pulse of heat to the surface of the sample in an air environment and during the occurance of thermal equilibrium the same probe detects the change in temperature. This change in temperature is related to the thermal conductivity of the sample. Since the thermal conductivity of natural diamond is at least an order of magnitude greater than a simulated diamond, such as cubic zirconia, it is readily detected. This method, however, is not sensitive enough to detect the slight change in thermal conductivity between Grade 8 and Grade 8.2 fasteners.
Another method of identifying materials nondestructively is disclosed in U.S. Pat. No. 2,924,771 issued to Elmer H. Greenberg et al. This invention teaches an improvement in the employment of the thermoelectric effect to identify a specimen of material and in particular metallic materials. This method utilizes a pair of electrically connected metallic contact members engaged with a sample specimen, wherein one contact member, the probe, provides the hot junction and at a slightly different region the other contact provides the cold junction. Thus, a thermoelectric voltage reading is generated which is claimed to identify the material.
The disadvantages of the above mentioned test method are numerous, with the major ones listed by the following comments:
a. Reproductibility of the reading is dependent upon the relative thermal conductivity and diffusivity of the probe and sample, as well as a properly proportioned probe geometry and contact area; and use of proper probe metal. In addition, resharpening of the probe with use is necessary.
b. Means have to be provided to minimize variable radiation losses in the contact members, otherwise the test becomes highly inaccurate. This necessitates calibration and control of additional heat to the system by the operator.
c. Optimum sensitivity is attained for different metals by previous tests of known specimens. Certain readings may be indicative of two or more metals. The ambiguity may be resolved by a single probe test wherein such results are then compared to a listing for that probe and may in all probability identify the specimen, if the diffusivities and thermoelectric effects are not too close.
d. Resultling test readings must be compared to a table of materials, each of known chemical analysis and physical condition when read with this method of standardization.
It is obvious from the noted disadvantages associated with the method described in U.S. Pat. No. 2,924,771, that it is not operator simple, reproducibility and accuracy is dependent upon a priori knowledge of the sample material, and results may well be ambiguous, such that the sample could not be definitively identified.
Another example of a nondestructive test method is described in U.S. Pat. No. 3,981,175, which was issued to Ogden H. Hammond III and Francis I. Baratta (hereby expressly incorporated by reference). In accordance with that system, the device is a nondestructive counterfeit gold bar and silver bar detection system based upon heat transfer principles. The principle entails the application of identical finite suddenly applied controlled heat pulses at a first region of a gold or silver bar of known purity, used as a standard, and the test bar of the same dimensions. The system is enclosed in an insulating medium. The temperatures are measured at a second region of each bar, which are not only dependent upon the thermal properties of each bar, but upon the time. Specifically, those thermal properties which are tested by this devise are specific heat, thermal conductivity, and density; and the combination of these properties known as diffusivity. Since these properties in gold and silver are unique, the temperature at the second region, specifically the end opposite from that which is suddenly pulsed by a quantity of heat will be at a higher temperature in a given time than that of any bar less pure than the standard gold or silver bar. Because of the large differences in thermal properties of gold and an alloyed gold sample, temperature measurements will reveal differences. However, the thermal properties of Grade 8 and 8.2 bolts are very similar and temperature determinations at their far ends will not guarantee discrimination.
Yet another example of a nondestructive test method to detect fraudulent precious metal bars is revealed in U.S. Pat. No. 4,381,154, issued to Ogden H. Hammond, III. It was found that of all possible forgeries, a non-alloyed tungsten forgery of gold, i.e., an insert of tungsten within the gold bar, is the most difficult to detect because the density and heat-capacity of tungsten and gold are virtually identical (a less difficult forgery to detect is an alloyed forgery wherein its composition is generally uniform throughout). Thus, an improvement in accuracy over the previous U.S. Pat. No. 3,981,175 was required. This improvement consists mainly of increasing the accuracy of the detection system by providing and controlling heat into the test chamber resulting in equilibrium, termed dynamic insulation; accurate heater control and using a compensated infrared sensor to measure the temperature at the far end opposite the heated end of the sample.
Although the improved techniques adopted in U.S. Pat. No. 4,381,154 will enhance the sensitivity of this test method it requires additional temperature sensors, controls and electronic instrumentation as compared to the method prescribed in U.S. Pat. No. 3,981,175.
Because the thermal properties of ferrous materials are not as unique as gold or silver, simple improvements of U.S. Pat. No. 3,981,175 are required to nondestructively detect counterfeit, mismarked, and substandard steel fasteners in a viable manner.